Relative wire stiffness due to lingual versus labial interbracket distance.

Orthodontic treatment of patients with lingual appliances necessitates modifications of the conventional mechanotherapy used with labial appliances. When the brackets are placed on the lingual instead of the labial surface, the interbracket distances in the anterior region of the arches are decreased significantly. The purpose of this study was to determine by how much these interbracket distances are decreased, and to quantitate what effect this has on relative wire stiffness. Lingual and labial brackets were placed on models of 30 orthodontically treated patients and a ratio comparing the lingual to labial interbracket distances was calculated to be 1:1.47. Twenty-five orthodontic wires at lengths of 1 inch and 1.47 inches were then evaluated for their stiffness in first-, second-, and third-order bending. The results showed that the decreased interbracket distance associated with lingual appliances makes a wire seem approximately 3 times as stiff as when used with labial appliances for first- and second-order bends, and approximately 1 1/2 times as stiff for third-order bends. Wire equivalence charts were developed to help orthodontists make direct comparisons between labial and lingual arch wires in regard to stiffness. These charts should prove useful to the clinician during initial alignment and final detailing when adequate arch wire flexibility is essential.